Every year hundreds of thousands of people develop medical complications such as nerve injury when hypodermic needles penetrate deeper than they should. A novel needle devised by researchers at Harvard Medical School and their colleagues automatically stops itself from going too far. The force from the first push of the device's plunger goes only to a blunt, flexible wire inside the hollow needle. As long as this filament remains unbent, a special clutch keeps the rest of the needle from advancing. On encountering resistance from tissue, the wire buckles and the clutch permits the entire needle to move forward. On reaching a target cavity, such as a blood vessel, the filament no longer faces resistance and so straightens out, preventing the needle from proceeding but uncovering the tip to allow medicine out. Described in the April 7 Proceedings of the National Academy of Sciences USA, the needle might reach clinics in three to five years.